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ou have seen several kinds of

plants and animals. However,

there are other living organisms

around us which we normally cannot

see. These are called

microorganisms

microbes. For example, you might

have observed that during the rainy

season moist bread gets spoilt and its

surface gets covered with greyish white

patches. Observe these patches through

a magnifying glass. You will see tiny,

black rounded structures. Do you know

what these structures are and where do

these come from?

2.1 Microorganisms

Activity 2.1

Collect some moist soil from the

field in a beaker and add water to

it. After the soil particles have

settled down, observe a drop of

water from the beaker under a

microscope. What do you see ?

Activity 2.2

Take a few drops of water from a

pond. Spread on a glass slide and

observe through a microscope.

Do you find tiny organisms moving

around?

These observations show that water

and soil are full of tiny organisms,

though not all of them fall into the

category of microbes. These

microorganisms or microbes are so

small in size that they cannot be seen

with the unaided eye. Some of these,

such as the fungus that grows on bread,

can be seen with a magnifying glass.

Others cannot be seen without the help

of a microscope. That is why these are

called microorganisms or microbes.

Microorganisms are classified into

four major groups. These groups are

bacteria, fungi, protozoa and some

algae. Some of these common

microorganisms are shown in

Figs. 2.1 - 2.4.

Viruses are also microscopic but are

different from other microorganisms.

They, however, reproduce only inside the

cells of the host organism, which may be

a bacterium, plant or animal. Some of the

viruses are shown in Fig. 2.5. Common

ailments like cold, influenza (flu) and

most coughs are caused by viruses.

Serious diseases like polio and chicken

pox are also caused by viruses.

Diseases like dysentery and malaria

are caused by protozoa(protozoans)

whereas typhoid and tuberculosis (TB)

are bacterial diseases.

You have learnt about some of

these microorganisms in Classes VI

and VII.

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Fig. 2.4 : Fungi

Fig. 2.1: Bacteria

Chlamydomonas

Spirogyra

Fig. 2.2 : Algae

Amoeba

Paramecium

Fig. 2.3 : Protozoa

Bread mould

Penicillium

Aspergillus

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2.2 Where do Microorganisms

Live?

Microorganisms may be single-celled

like bacteria, some algae and protozoa,

or multicellular, such as many algae and

fungi. They live in all types of

environment, ranging from ice cold

climate to hot springs; and deserts to

marshy lands. They are also found

inside the bodies of animals including

humans. Some microorganisms

grow on other organisms while others

exist freely.

2.3 Microorganisms and Us

Microorganisms play an important role

in our lives. Some of them are beneficial

in many ways whereas some others are

harmful and cause diseases. Let us

study about them in detail.

Microorganisms have been used for

the production of alcohol since ages.

Friendly Microorganisms

Microorganisms are used for various

purposes. They are used in the

preparation of curd, bread and cake.

Curd contains several micro-

organisms. Of these, the bacterium,

Lactobacillus

promotes the formation

of curd. It multiplies in milk and

converts it into curd. Bacteria are also

involved in the making of cheese,

pickles and many other food items. An

important ingredient of rava (sooji)

idlis

and

bhaturas is curd. Can you

guess why? Bacteria and yeast are also

helpful for fermentation of rice idlis

and dosa batter.

Fig. 2.5 : Viruses

I saw that my mother

added a little curd to warm

milk to set curd for the

next day. I wonder why?

They are also used in cleaning up

of the environment. For example, the

organic wastes (vegetable peels, remains

of animals, faeces, etc.) are broken

down into harmless and usable

substances by bacteria. Recall that

bacteria are also used in the

preparation of medicines. In agriculture

they are used to increase soil fertility

by fixing nitrogen.

Making of Curd and Bread

You have learnt in Class VII that milk is

turned into curd by bacteria.

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spoon of yeast powder to the sugar

solution. Keep it covered in a warm

place for 4-5 hours. Now smell the

solution. Can you get a smell?

Louis Pasteur

discovered

fermentation

in 1857.

In 1929, Alexander

Fleming was working

on a culture of disease-

causing bacteria.

Suddenly he found the

spores of a little green mould in one of

his culture plates. He observed that

the presence of mould prevented the

growth of bacteria. In fact, it also killed

many of these bacteria. From this the

mould penicillin was prepared.

This is the smell of alcohol as sugar

has been converted into alcohol by yeast.

This process of conversion of sugar into

alcohol is known as fermentation.

Activity 2.3

Take ½ kg flour (atta or maida),

add some sugar and mix with

warm water. Add a small amount

of yeast powder and knead to

make a soft dough. What do you

observe after two hours? Did you

find the dough rising?

Maida with yeast powder

Raised maida

Fig. 2.6

Yeast reproduces rapidly and

produces carbon dioxide during

respiration. Bubbles of the gas fill the

dough and increase its volume (Fig. 2.6).

This is the basis of the use of yeast in

the baking industry for making breads,

pastries and cakes.

Commercial Use of Microorganisms

Microorganisms are used for the large scale

production of alcohol, wine and acetic acid

(vinegar). Yeast is used for commercial

production of alcohol and wine. For this

purpose yeast is grown on natural sugars

present in grains like barley, wheat, rice,

crushed fruit juices, etc.

Activity 2.4

Take a 500 mL beaker filled upto

¾ with water. Dissolve 2-3

teaspoons of sugar in it. Add half a

Medicinal Use of Microorganisms

Whenever you fall ill the doctor may

give you some antibiotic tablets,

capsules or injections such as of

penicillin. The source of these

medicines is microorganisms. These

medicines kill or stop the growth of the

disease-causing microorganisms. Such

medicines are called antibiotics. These

days a number of antibiotics are being

produced from bacteria and fungi.

Streptomycin, tetracycline and

erythromycin are some of the

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commonly known antibiotics which are

made from fungi and bacteria. The

antibiotics are manufactured by

growing specific microorganisms and

are used to cure a variety of diseases.

Antibiotics are even mixed with the

feed of livestock and poultry to check

microbial infection in animals. They are

also used to control many plant

diseases.

It is important to remember that

antibiotics should be taken only on

the advice of a qualified doctor. Also

you must complete the course

prescribed by the doctor. If you take

antibiotics when not needed or in

wrong doses, it may make the drug

less effective when you might need

it in future. Also antibiotics taken

unnecessarily may kill the beneficial

bacteria in the body. Antibiotics,

however, are not effective against

cold and flu as these are caused by

viruses.

Vaccine

from the disease-causing microbes for

ever. This is how a vaccine works.

Several diseases, including cholera,

tuberculosis, smallpox and hepatitis can

be prevented by vaccination.

Edward Jenner

discovered the

vaccine for small-

pox in 1798.

Why are children/infants

given vaccination?

When a disease-carrying microbe enters

our body, the body produces antibodies

to fight the invader. The body also

remembers how to fight the microbe if

it enters again. If dead or weakened

microbes are introduced into a healthy

body, the body fights and kills

the invading bacteria by producing

suitable antibodies. The antibodies

remain in the body and we are protected

In your childhood, you must have

been given injections to protect

yourself against several diseases.

Can you prepare a list of these

diseases? You may take help from

your parents.

It is essential to protect all children

against these diseases. Necessary

vaccines are available in the nearby

hospitals. You might have seen the

advertisement on TV and newspapers

regarding protection of children against

polio under the Pulse Polio Programme.

Polio drops given to children are

actually a vaccine.

A worldwide campaign against

smallpox has finally led to its eradication

from most parts of the world.

These days vaccines are made on a

large scale from microorganisms to

protect humans and other animals from

several diseases.

Increasing Soil Fertility

Some bacteria (Fig. 2.7) are able to fix

nitrogen from the atmosphere to enrich

soil with nitrogen and increase its

fertility. These microbes are commonly

called biological nitrogen fixers.

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Cleaning the Environment

Boojho and Paheli had observed the

school gardener making manure. Along

with their friends, they collected wastes

of plants, vegetables and fruits from

nearby houses and gardens. They put

them in a pit meant for waste disposal.

After some time, it decomposed and got

converted to manure. Boojho and

Paheli wanted to know how this had

happened.

Activity 2.5

Take two pots and fill each pot half

with soil. Mark them A and B. Put

plant waste in pot A and things like

polythene bags, empty glass bottles

and broken plastic toys in pot B.

Put the pots aside. Observe them

after 3-4 weeks.

Do you find any difference in the

contents of the two pots? If so, what is

the difference? You will find that plant

waste in pot A, has been decomposed.

How did this happen? The plant waste

has been converted into manure by the

action of microbes. The nutrients

released in the process can be used by

the plants again.

Did you notice that in pot B, the

polythene bags, empty glasses, bottles

and broken toy parts did not undergo

any such change? The microbes could

not ‘act’ on them and convert them into

manure.

You often see large amounts of

dead organic matter in the form of

decaying plants and sometimes dead

animals on the ground. Y

ou find

that they disappear after some time.

This is because the microorganisms

decompose dead organic waste of plants

and animals converting them into

simple substances. These substances

are again used by other plants and

animals. Thus, microorganisms can

be used to degrade the harmful and

smelly substances and thereby clean

up the environment.

2.4 Harmful Microorganisms

Microorganisms are harmful in many

ways. Some of the microorganisms

cause diseases in human beings, plants

and animals. Such disease-causing

Fig. 2.7 : The Nitrogen fixing cyanobacteria (blue green algae)

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microorganisms are called pathogens.

Some microorganisms spoil food,

clothing and leather. Let us study more

about their harmful activities.

Disease causing Microorganisms

in Humans

Pathogens enter our body through the

air we breathe, the water we drink or

the food we eat. They can also get

transmitted by direct contact with an

infected person or carried by an animal.

Microbial diseases that can spread from

an infected person to a healthy person

through air, water, food or physical

contact are called communicable

diseases. Examples of such diseases

include cholera, common cold, chicken

pox and tuberculosis.

When a person suffering from

common cold sneezes, fine droplets of

moisture carrying thousands of viruses

are spread in the air. The virus may enter

the body of a healthy person while

breathing and cause infection.

There are some insects and animals

which act as carriers of disease-

causing microbes. Housefly is one such

carrier. The flies sit on the garbage and

animal excreta. Pathogens stick to their

bodies. When these flies sit on uncovered

food they may transfer the pathogens.

Whoever eats the contaminated food is

likely to get sick. So, it is advisable to

always keep food covered. Avoid

consuming uncovered items of food.

Another example of a carrier is the

female Anopheles mosquito (Fig. 2.8),

which carries the parasite of malaria

(Plasmodium). Female Aedes mosquito

acts as carrier of dengue virus.

How can we control the spread of malaria

or dengue?

Then how do you

prevent the spread of

communicable

diseases?

We should keep a

handkerchief on the

nose and mouth while

sneezing. It is better to

keep a distance from

infected persons.

Fig. 2.8 : Female Anopheles mosquito

Why does the teacher

keep telling us not

to let water collect

anywhere in the

neighbourhood?

All mosquitoes breed in water. Hence,

one should not let water collect

anywhere, in coolers, tyres, flower pot,

etc. By keeping the surroundings clean

and dry we can prevent mosquitoes from

breeding. Try to make a list of measures

which help to avoid the spread of

malaria.

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Some of the common diseases

affecting humans, their mode of

transmission and few general methods

of prevention are shown in Table 2.1.

Disease causing Microorganisms

in Animals

Several microorganisms not only cause

diseases in humans and plants, but also

in other animals. For example, anthrax

is a dangerous human and cattle

disease caused by a bacterium. Foot

and mouth disease of cattle is caused

by a virus.

Disease causing Microorganisms

in Plants

Several microorganisms cause

diseases in plants like wheat, rice, potato,

sugarcane, orange, apple and others.

The diseases reduce the yield of crops.

See Table 2.2 for some such plant

diseases. They can be controlled by the

Human Disease Causative Mode of Preventive Measures

Microorganism Transmission (General)

Tuberculosis Bacteria Air

Measles Virus Air

Chicken Pox Virus Air/Contact

Polio Virus Air/Water

Cholera Bacteria Water/Food

Typhoid Bacteria Water

Hepatitis A Virus Water

Malaria Protozoa Mosquito

Robert Köch (1876)

discovered the bacterium

(Bacillus anthracis) which

causes anthrax disease.

Keep the patient in complete

isolation. Keep the personal

belongings of the patient away

from those of the others.

Vaccination to be given at

suitable age.

Maintain personal hygiene

and good sanitary habits.

Consume properly cooked food

and boiled drinking water.

Vaccination.

Drink boiled drinking water.

Vaccination.

Use mosquito net and

repellents. Spray insecticides

and control breeding of

mosquitoes by not allowing

water to collect in the

surroundings.

Table 2.1: Some Common Human Diseases caused by Microorganisms

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use of certain chemicals which kill the

microbes.

Food Poisoning

Boojho was invited by his friend to a

party and he ate a variety of foodstuff.

On reaching home he started vomiting

and had to be taken to a hospital. The

doctor said that this condition could be

due to food poisoning.

make the food poisonous causing

serious illness and even death. So, it

is very important that we preserve

food to prevent it from being spoilt.

2.5 Food Preservation

In Chapter 1, we have learnt about the

methods used to preserve and store food

grains. How do we preserve cooked food

at home? You know that bread left

unused under moist conditions is

attacked by fungus. Microorganisms

spoil our food. Spoiled food emits bad

smell and has a bad taste and changed

colour. Is spoiling of food a chemical

reaction?

Paheli bought some mangoes but she

could not eat them for a few days. Later

she found that they were spoilt and

rotten. But she knows that the mango

pickle her grandmother makes does not

spoil for a long time. She is confused.

Plant Micro- Mode of

Diseases organism Transmission

Citrus Bacteria Air

canker

Rust of Fungi Air,

wheat seeds

Yellow vein Virus Insect

mosaic of

bhindi (Okra)

Paheli wonders how

food can become a

‘poison’.

Table 2.2: Some Common Plant Diseases caused by Microorganisms

Food poisoning could be due to the

consumption of food spoilt by some

microorganisms. Microorganisms

that grow on our food sometimes

produce toxic substances. These

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Let us study the common methods

of preserving food in our homes. We

have to save it from the attack of

microorganisms.

Chemical Method

Salts and edible oils are the common

chemicals generally used to check the

growth of microorganisms. Therefore

they are called preservatives. We add

salt or acid preservatives to pickles to

prevent the attack of microbes. Sodium

benzoate and sodium metabisulphite are

common preservatives. These are also

used in jams and squashes to check

their spoilage.

Preservation by Common Salt

Common salt has been used to preserve

meat and fish for ages. Meat and fish

are covered with dry salt to check

the growth of bacteria. Salting is also

used to preserve amla

, raw mangoes,

tamarind, etc.

Preservation by Sugar

Jams, jellies and squashes are preserved

by sugar. Sugar reduces the moisture

content which inhibits the growth of

bacteria which spoil food.

Preservation by Oil and Vinegar

Use of oil and vinegar prevents spoilage

of pickles because bacteria cannot live

in such an environment. Vegetables,

fruits, fish and meat are often preserved

by this method.

Heat and Cold Treatments

You must have observed your mother

boiling milk before it is stored or used.

Boiling kills many microorganisms.

Similarly, we keep our food in the

refrigerator. Low temperature inhibits

the growth of microbes.

Pasteurised milk can be consumed

without boiling as it is free from harmful

microbes. The milk is heated to about

C for 15 to 30 seconds and then

suddenly chilled and stored. By doing

so, it prevents the growth of microbes.

This process was discovered by Louis

Pasteur. It is called pasteurisation.

Storage and Packing

These days dry fruits and even

vegetables are sold in sealed air tight

packets to prevent the attack of

microbes.

2.6 Nitrogen Fixation

You have learnt about the bacterium

Rhizobium in Classes VI and VII. It is

involved in the fixation of nitrogen in

leguminous plants (pulses). Recall that

Rhizobium lives in the root nodules of

leguminous plants (Fig. 2.9), such as

beans and peas, with which it has a

symbiotic relationship. Sometimes

nitrogen gets fixed through the action

of lightning. But you know that the

amount of nitrogen in the atmosphere

remains constant. You may wonder

how? Let us understand this in the next

section.

Why does the milk that

comes in packets not spoil?

My mother told me that the

milk is ‘pasteurised’. What

is pasteurisation?

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nitrogen cannot be taken directly by

plants and animals. Certain bacteria and

blue green algae present in the soil fix

nitrogen from the atmosphere and

convert it into compounds of nitrogen.

Once nitrogen is converted into these

usable compounds, it can be utilised by

plants from the soil through their root

system. Nitrogen is then used for the

synthesis of plant proteins and other

compounds. Animals feeding on plants

get these proteins and other nitrogen

compounds (Fig. 2.10).

When plants and animals die,

bacteria and fungi present in the soil

convert the nitrogenous wastes into

nitrogenous compounds to be used by

plants again. Certain other bacteria

convert some part of them to nitrogen

gas which goes back into the

atmosphere. As a result, the percentage

of nitrogen in the atmosphere remains

more or less constant.

Fig. 2.10 : Nitrogen cycle

Fig. 2.9 : Roots of a leguminous plant with root

nodules

2.7 Nitrogen cycle

Our atmosphere has 78% nitrogen gas.

Nitrogen is one of the essential

constituents of all living organisms as

part of proteins, chlorophyll, nucleic

acids and vitamins. The atmospheric

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KEYWORDS KEYWORDS

KEYWORDS

ALGAE

ANTIBIOTICS

ANTIBODIES

BACTERIA

CARRIER

COMMUNICABLE

DISEASES

FERMENTATION

FUNGI

LACTOBACILLUS

MICROORGANISM

NITROGEN CYCLE

NITROGEN FIXATION

PASTEURISATION

PATHOGEN

PRESERVATION

PROTOZOA

RHIZOBIUM

VACCINE

VIRUS

YEAST

WHA WHA

WHA

T YT Y

T Y

OU HAOU HA

OU HA

VE LEARNTVE LEARNT

VE LEARNT

Ü Microorganisms are too small and are not

visible to the unaided eye.

Ü They can live in all kinds of environment,

ranging from ice cold climate to hot springs

and deserts to marshy lands.

Ü Microorganisms are found in air, water and

in the bodies of plants and animals.

Ü They may be unicellular or multicellular.

Ü Microorganisms include bacteria, fungi,

protozoa and some algae. Viruses, though

different from the above mentioned living

organisms, are considered microbes.

Ü Viruses are quite different from other

microorganisms. They reproduce only inside

the host organism: bacterium, plant or ani-

mal cell.

Ü Some microorganisms are useful for

commercial production of medicines and

alcohol.

Ü Some microorganisms decompose the organic

waste and dead plants and animals into simple

substances and clean up the environment.

Ü Protozoans cause serious diseases like

dysentery and malaria.

Ü Some of the microorganisms grow on our food

and cause food poisoning.

Ü Some microorganisms reside in the root

nodules of leguminous plants. They can fix

nitrogen from air into soil and increase the

soil fertility.

Ü Some bacteria present in the soil fix nitrogen

from the atmosphere and convert into

nitrogenous compounds.

Ü Certain bacteria convert compounds of nitro-

gen present in the soil into nitrogen gas which

is released to the atmosphere.

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Exercises

1. Fill in the blanks.

(a) Microorganisms can be seen with the help of a ____________.

(b) Blue green algae fix __________ directly from air and enhance fertility

of soil.

(d) Cholera is caused by __________.

2. Tick the correct answer.

(a) Yeast is used in the production of

(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen

(b) The following is an antibiotic

(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast

(i) female Anopheles mosquito (ii) cockroach

(iii) housefly (iv) butterfly

(d) The most common carrier of communicable diseases is

(i) ant (ii) housefly (iii) dragonfly (iv) spider

(e) The bread or idli dough rises because of

(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection

3. Match the organisms in Column A with their action in

Column B.

A B

(i) Bacteria (a) Fixing nitrogen

(ii) Rhizobium (b) Setting of curd

(iii) Lactobacillus (c) Baking of bread

(iv) Yeast (d) Causing malaria

(v) A protozoan (e) Causing cholera

(vi) A virus (f) Causing AIDS

(g) Producing antibodies

4. Can microorganisms be seen with the naked eye? If not, how can they be

seen?

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5. What are the major groups of microorganisms?

6. Name the microorganisms which can fix atmospheric nitrogen in the soil.

7. Write 10 lines on the usefulness of microorganisms in our lives.

8. Write a short paragraph on the harmful effects of microorganisms.

9. What are antibiotics? What precautions must be taken while taking

antibiotics?

E X E R C I S E S

Extended Learning — Activities and Projects

1. Pull out a gram or bean plant from the field. Observe its roots. You

will find round structures called root nodules on the roots. Draw a

diagram of the root and show the root nodules.

2. Collect the labels from the bottles of jams and jellies. Write down

the list of contents printed on the labels.

3. Visit a doctor. Find out why antibiotics should not be overused.

Prepare a short report.

4. Project : Requirements – 2 test tubes, marker pen, sugar, yeast

powder, 2 balloons and lime water.

Take two test tubes and mark them A and B. Clamp these tubes in

a stand and fill the

m with water leaving some space at the top. Put

two spoonfuls of sugar in each

of the test tubes. Add a spoonful

of yeast in test tube B. Inflate the

two balloons incompletely. Now

tie the balloons on the mouths

of each test tube. Keep them in

a warm place, away from

sunlight. Watch the setup every

day for next 3-4 days. Record

your observations and think of

an explanation.

Now take another test tube

filled 1/4 with lime water.

Remove the balloon from test

tube B in such a manner that

gas inside the balloon does not

escape. Fit the balloon on the

test tube and shake well.

Observe and explain.

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Did You Know?

Bacteria have lived on the earth for much longer than human beings.

They are such hardy organisms that they can live under extreme conditions.

They have been found living in boiling mudpots and extremly cold icy waters.

They have been found in lakes of caustic soda and in pools of concentrated

sulphuric acid. They can survive at depths of several kilometres. They

probably can survive in space, too. A kind of bacterium was recovered

from a camera which stood on the moon for two years.There is probably no

environment in which bacteria cannot survive.

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